Transcript Table of Contents - Mr. Tobin`s Earth Science Class

Title: 17.2 Seafloor Spreading
Page #: 88
Date: 3/18/2012
 Students
will be able to summarize
evidence that led to the discovery of
seafloor spreading.
 Students will be able to explain the
significance of magnetic patterns on the
seafloor.
 Students will be able to explain the
process of seafloor spreading.
 Basalt: A
dark grey to black fine grained
igneous rock.
Idea:  Oceanic crust forms at mid-ocean
ridges and becomes part of the
 Pg. 473
seafloor.
 Main
 Mapping
The ocean
Floor:
Pg. 473
 Early
1900s: People believe that
oceans are carved out by erosion.
• Believe that ocean floor is flat and old.
 1940s: WWII
submarine warfare
results in the development of
SONAR technology.
• Also results in MAGNOMETER
technology.
 Mapping
the
Ocean
Floor
pg. 473
 Sonar: Echolocation.
Sub sends
sound out, it bounces off an object
and reveals its location.
 Magnometer: Device that can
detect changes in magnetic fields
(used to find enemy subs.)
Sonar
Magnetometer
 Mapping
the
Ocean
Floor
pg. 473
 Late
1940s-1950s: WWII ends.
Scientists use new technology to
study depth and topography of the
ocean floor (sonar.)
• Scientists study magnetic fields of
ocean floor rocks (magnometer.)
 Mapping
the
Ocean
Floor
pg. 474
 1947: Bruce
Heezen and Marie
Tharp of Columbia University
began to systematically map
Earth’s seafloor features.
 Heezen went out on research
cruises and gathered data from
1947 – 1965.
 Tharp: stayed on shore to construct
maps. (Women were not allowed
on research cruises.)
Heezen and Tharp
World Ocean Floor; Published 1974
 Mapping
the
Ocean
Floor
pg. 474
 Surprise
Findings!
1. Ridges: huge underwater mountain
ranges, 80,000 km long and up to 3
km high.


Largest mountain range on Earth.
Earthquakes and volcanoes are
common along ridges.
Mid-Atlantic
Ridge
 Mapping
the
Ocean
Floor
pg. 474

Surprise Findings!
2. Deep-Sea Trenches: Narrow
elongated depressions in sea floor.


Can be 100s of km long and many km
deep.
Example: Marianas Trench; Mt. Everest
(9 km above sea-level) can fit in
Marianas Trench with 9 Empire State
Buildings on it.
 Mapping
the
Ocean
Floor
pg. 474
•
1.
2.
3.
Ridges and trenches lead to more
questions:
What formed mountains?
What is source of valcanism on
mountain range?
What forces depressed Earth’s
crust to create huge trenches?
 Ocean
Rocks
and
Sediment
s
pg. 475

1.
2.
2 Discoveries from samples of
Deep Sea rocks and
sediments:
Age of rocks increases as you
get farther away from a midocean ridge – oldest rocks at
trenches.
Thickness of sediments
increases as you get farther
away from mid-ocean ridge
(older crust has more time to
collect sediments.)
 Ocean
Rocks
and
Sediment
s
pg. 475




Oceanic Rocks only 180
million years old.
Continental Rocks can be up
to 3.8 billion years old.
Layer of sediment on oceanic
crust is only a few hundred
meters thick.
Layer of sediment on
continental crust is 20
kilometers thick.
Thickness of sediments increases as you get farther from a
mid-ocean ridge.
 Ocean
Rocks
and
Sediment
s
pg. 475

Oceanic rocks and
sediments are symmetrical
in age and depth of
sediments (mirror images)
on either side of a mid-ocean
ridge.
 Magnetism
pg. 476
Earth has a magnetic field.

•
•
Causes compasses to point north.
A result of flow of molten iron in
liquid outer core of the earth.
Magnetic Reversal:

•
•
When Earth’s magnetic field
changes direction.
Happens when there is a change
in flow of the earth’s core.
 Magnetic
Polarity
Time Scale
pg. 476
Paleomagnetism: Study of
history of Earth’s magnetic
field.

•
Lava contains magnetite – when it
solidifies crystals point toward
the magnetic pole because of
Earth’s magnetic field.
Magnetic Time
Periods
 Magnetic
Symmetry
pg. 476
Magnetic Symmetry:
Oceanic crust is mostly
basaltic.

•

Contains large amounts of iron
bearing minerals.
Magnetometers revealed a
pattern in the ocean floor.
 Magnetic
Pattern in
Ocean Floor
pg. 476
Regions with normal and
regions with reverse polarity
form a series of stripes
across the floor parallel to
mid-ocean ridges.

•
Age and width of the stripes
matched from one side of the
ridge to the other.
Seafloor Striping
 Magnetic
Pattern in
Ocean Floor
pg. 477
Isochron: A line on a map that
shows points with the same
age.

•
Scientists were able to match
patterns of reversals on land to the
reversals on the sea floor to
determine the age of the ocean
floor.
 Seafloor

Spreading
pg. 479

Seafloor Spreading: Theory
that explains how new crust is
formed at ocean ridges and is
destroyed at deep-sea
trenches.
Steps:
1.
2.
3.
Less dense magma rises at ridges and some
solidifies.
Solid magma makes new seafloor.
Most magma gets turned horizontally away from
ridge under the crust.
 Seafloor
Spreading
pg. 479
Continental Drift questions
answered:

•
•
Continents don’t move through
oceanic crust.
Continents are carried as passengers
that ride along as ocean crust moves.